In measurements technology, a number of different measuring devices are known for determining or monitoring one or more process variables of a medium. For determining the reaching of a predetermined fill level or for monitoring a minimum or maximum fill level of a liquid in a container, among others, vibronic measuring devices are applied, placed at the height in the container to be monitored.
As a rule, vibronic measuring devices have two fork-like tines, which are excited to opposite phase oscillations at the resonance frequency by a drive unit via a membrane. Likewise, so called membrane oscillators without an additional oscillatable unit, or single rods are known. The drive occurs, in such case, via piezoelectric elements. If the oscillatory system is covered by the measured medium, the oscillation is damped and the oscillation frequency decreases, whereby the reaching of the limit level is signaled. Such vibronic measuring devices having an oscillatory fork for fill level detection of liquids are developed by the assignee under the mark LIQUIPHANT and are available and in a great variety of embodiments. The construction of a LIQUIPHANT measuring device is described, for example, in the document EP 1261437 B1.
The density of the measured medium can also be determined with said vibronic measuring devices. The higher the density of a liquid, the lower is the resonance frequency, with which the oscillatory system oscillates. The resonance frequency is, however, temperature dependent, so the temperature of the medium must also be determined for a density determination. Besides this application, a number of additional applications are known, in which, besides the fill level, the determining and monitoring of the process temperature is also required, for example, for detecting the reaching of a maximum allowable process temperature for a sensor.
Until now, a temperature measurement has been possible, for example, by bringing an additional temperature sensor into contact with the container externally, i.e. outside of the fill level or density measuring device, via a separate process connection. For a density measurement compensated for temperature, the temperature sensor and the density measuring device have been connected to the evaluation computer. Each additional process connection poses an additional risk relative to the seal and hygiene of the process, so that it is desirable to integrate the temperature measurement in a measuring device required anyway, and thus save an additional process connection.
Integration of the temperature sensor in a process-near area of the sensor housing of a vibronic fill level measuring device is difficult due to the way in which the piezoelectric driving/receiving unit is mounted. This driving/receiving unit is introduced in the sensor housing on the side of the sensor housing, from which the oscillatable unit faces away. If the temperature sensor is secured on the housing wall of the sensor, it forms, in such case, a hindrance, e.g. for the mounting of the piezoelements. The further the temperature sensor is removed from the process, however, the greater the temperature determined by it deviates from the process temperature.
Document DE 102006007199 A1 describes a vibration-type limit switch, in which a temperature determining unit is introduced between the elements of the transmitting/receiving unit. In the still unpublished German patent application 102009029490.2, temperature sensors are introduced in process-near elements of a fill level measuring device. The disadvantage of these solutions is that they complicate the manufacture of the measuring device.